PROJECT SUMMARY/ABSTRACT The US Surgeon General reports that skin cancer is the most common type of cancer in the US. The number of Americans who have had skin cancer at some point over the last three decades is estimated to exceed all other cancers combined. There are more than 75,000 new cases of melanoma each year, resulting in more than 10,000 deaths. Reports indicate that nearly 90% of skin cancers are caused by UV exposure, and despite the risks many Americans continue to subject themselves to harmful levels of UV partly because it is burdensome to accurately assess one's UV dose. Furthermore, the experience of sunburn or even melanoma often do not provide sufficient motivation to change sun behavior. Wearable UV monitors offer potential remedy, but existing devices are expensive, obtrusive, and often incompatible with water recreation. Their intrinsic operational mode has inaccuracy derived from intermittent measurements and interpolative algorithms to infer dose. These shortcomings cause non-adoption, improper use or discontinuation, limiting their ability to prevent overexposure, sunburn, and skin cancer. A critical unmet need is for a low cost, unobtrusive sun protection system that combines accurate dosimetry with user-responsive software interfaces capable of influencing healthy behavior. The proposed work addresses the need with a breakthrough sensor technology and an innovative user interface that enables informed sun-protection behavior. The sensors are pea-sized battery-free ?stickers? with fundamentally differentiated modes of use, cost structures, and accuracy. Uninterrupted charge accumulation imparts continuous UV-A and UV-B measurement capability with greater accuracy than any alternative. The result is a personal dosimeter with vanishingly small obtrusiveness and correspondingly small cost, allowing ?giveaway? distribution strategies. Real-time messages triggered by user-specific exposure will promote healthy sun behavior in the general population and at-risk segments, including those with medicine-induced photo- sensitivities or diseases that increase sensitivity to the sun. Activities in Phase I will establish the accuracy of the sensors through systematic comparisons to (non- wearable) laboratory instrumentation and produce a mobile app with bi-directional communications based on UV tolerances specific to the user's skin type. Focus groups will assess the usability of the system and its likelihood to influence sun-protection behavior. Phase 2 activities will optimize designs for cost and reliability in real-world conditions, with targets of <$2 per device and robust operation during/after watersports and heavy sweating. Successful completion of these tasks will enable assessment of the technology in prospective cohort studies in high-risk populations of melanoma survivors aged 18-49. Extended (90 day) behavior studies of 100 individuals will determine compliance (target >85%) and user-satisfaction (target >70%) to confirm broad acceptability and efficacy in influencing healthy sun behavior.